The present invention relates generally to a method of and apparatus for receiving a signal from a wideband digital source in the presence of a narrow band interfering source, e.g., from a geosynchronous satellite in close arcuate proximity to another geosynchronous satellite and, more particularly, to such a method and apparatus wherein the receiver is responsive to signals from both sources and the desired signal is discriminated from the undesired signal by a spectral analysis even though the narrow band signal has substantial frequency components in the frequency band of the wide band source.
There are certain situations wherein a relatively wide band first signal containing digitally modulated symbols is desirably received in the presence of a relatively narrow band analog second signal occupying a frequency band that is in the wide frequency band of the first signal. For example, geosynchronous satellites containing transponders for C-band television emissions are typically spaced from each other by 2xc2x0 of orbital arc and emit 5 to 10 watts. In the early to mid 1980""s, a 120xc2x0 K low noise block (LNB) downconverter was typical for terrestrial reception of analog television signals having video information frequency modulated on a C-band carrier emitted from the satellite. To bring the carrier to noise ratio (C/N) of the terrestrially received signals (typically about 11 dB) to acceptable levels, terrestrial antennas having parabolic reflector dishes with a 10xe2x80x2 diameter were used. A 10xe2x80x2 dish has a 3 dB beamwidth of 1.7xc2x0 (i.e. the 3 dB power points of the antenna power pattern are 0.85xc2x0 from the dish boresight axis); the pattern has a first null 2.3xc2x0 from the dish boresight axis. Because the satellites are spaced 2xc2x0 apart, the 10xe2x80x2 diameter dishes protect against adjacent satellites by receiving the signals from the adjacent satellites on the low gain side lobes of the antenna pattern. Typically, the antenna gain of the side lobes of a good parabolic reflector dish and feedhorn coupled with the reflector is 18 dB below the boresight gain. Thus a high gain antenna automatically protects the desired signal coming from the satellite along the antenna boresight axis from interference from other adjacent satellite signals. Based on this reasoning C-Band satellites are placed 2xc2x0 apart in the orbital arc.
Technology, however, has advanced to a point where, if only thermal noise were the dominating problem, the diameter of the reflector dish could be reduced. Consumer television receivers for receiving analog video signals are overdesigned to protect against interference from adjacent direct broadcast satellites. New technologies have led to development of much lower noise LNB""s with noise temperatures as low as 25xc2x0 K. For example, a 7xe2x80x2 dish driving a LNB downconverter having a 40xc2x0 K noise temperature has the same noise performance as a 10xc2x0 dish driving a 120xc2x0 K LNB downconverter.
However, satellites recently put into orbit or soon to be in orbit operate at maximum permissible power levels of 40 dB equivalent radiated power (EIRP) over most of the United States. When such a power level is factored into the C/N calculation it is found that a 4xe2x80x2 diameter dish suffices for reception of analog C-Band f.m. video (C/N=10 dB). The 3 dB beamwidth of a 4xe2x80x2 dish at C Band is 4.2xc2x0; the first null is 5.8xc2x0 from boresight. Thus, the main lobe of an antenna having a 4xe2x80x2 dish can span as many as four satellites that are 2xc2x0 apart. Generally speaking, satellites which are 4xc2x0 apart have identical frequency/polarization plans, leading to the possibility of significant interference from a satellite that is two positions away in the orbital arc from a satellite aligned with the dish boresight axis. This interference has not been conveniently filtered from the desired signal with a fixed parameter filter. This is true even though the spectrum of the unwanted signal is specified only as being in the bandwidth of the desired signal.
The United States Federal Communications Commission frequency/polarization plan requires satellite transponders having adjacent center frequencies on satellites 2xc2x0 apart to emit radiation having the same polarization. Since signals from these identically polarized transponders can and do overlap in frequency, signals from adjacent satellites also interfere with each other when they are both received by an antenna having a relatively wide beamwidth. Such an interfering signal cannot be conveniently filtered from the desired signal
A conclusion that can be drawn from C-band transmission of analog video is that if thermal noise, rather than interference were the governing consideration, a smaller dish would be feasible. This conclusion has even greater validity for direct satellite broadcasting of digital video signals. Professional grade satellite C-band f.m. video receivers have an i.f. bandwidth of 30 MHz. In various scenarios the information rate of digital video relayed through a satellite transponder is 30 Mb/s. If the LNB, antenna size and satellite EIRP are equal for 30 Mb/s digital video and 30 MHz f.m. analog video signals, the carrier to noise ratio (C/N) for the analog case equals energy per bit (Eb/N0) for the digital case; Eb/N0 is a measure of noise density and thus is similar to signal to noise ratio (S/N) of an analog signal. However the Eb/N0 required for satellite transmission of C-band f.m. video, assuming concatenated forward error correction coding consisting of a Reed Solomon coder and a high rate convolutional encoder, is 5.5 dB, not 11 dB, as is typical of the C/N for analog video. By applying the increased margin to dish diameter, a 4xe2x80x2 dish can be reduced to a 2.4xe2x80x2 dish, if thermal noise were the only consideration. Of course, the smaller dish provides even less spatial discrimination against adjacent satellites so interference from other satellites becomes more of a problem.
Small diameter dishes are important because they increase the number of potential terrestrial installations (particularly homes) of receivers for direct satellite broadcasting via digital modulation. A current K-band DBS (direct broadcast satellite) system is viable because it employs a one-and-a-half foot (i.e. about 0.5 meters) diameter dish that is unobtrusive, has low wind resistance and provides acceptable reception. A C-Band DBS (Direct Broadcast Satellite) service is expected to be more profitable and certainly viable in areas having small receiver dish diameters because fixed costs thereof are spread over a base of cable subscribers and residences having consumer television receivers responsive to signals derived from a down link having digital modulation.
A proposed solution to reduce interference is to increase the 2xc2x0 spacing between adjacent satellites. This does not seem to be feasible or likely. Hence, the interference problem between signals from adjacent satellites is most acute for C-band digital television. A technical solution is preferable to a political solution because it does not require coordination between satellite operators or compromises between the organizations involved.
An interference situation similar to that of small diameter dishes responsive to adjacent satellites can also exist for: (1) quadrature amplitude modulation (QAM) cable transmission of video wherein it is desirable for (a) a wide band digitally modulated signal containing television broadcast information and (b) a narrow band analog signal containing television broadcast information to occupy the same frequency band on the same cable conductors, and (2) terrestrial transmission through the air of (a) multilevel vestigial sideband high definition television information having a relatively wide bandwidth and (b) a narrow bandwidth analog television broadcast signal that is in the same frequency band as the high definition wide bandwidth information.
It is accordingly an object of the present invention to provide a new and improved method of and apparatus for receiving a wideband digital signal in the presence of a narrow band interfering signal wherein the narrow band signal occupies a part of the same bandwidth as the wideband signal and has a variable center frequency.
Another object of the present invention is to provide a new and improved method of and apparatus for receiving a desired signal from a geosynchronous satellite having close arcuate spacing with another geosynchronous satellite, wherein one of the satellites emits narrow band electromagnetic energy that occupies a part of a wide frequency band of electromagnetic energy of a desired signal, emitted from another satellite, wherein the energy from both satellites is incident on an antenna dish having a beamwidth large enough to receive electromagnetic energy from both satellites.
In accordance with one aspect of the invention, a desired wide band signal having a predetermined carrier frequency and including sequential symbols representing digital values is discriminated from a narrow band interfering signal having a carrier frequency in the wide band by initially applying both of the signals to a filter so both signals are passed to an output of the filter. In response to the filter output an estimate of the symbol value of the desired wideband signal is derived and compared with the filter output to derive an error representing signal. A characteristic of the filter is controlled in response to the error representing signal to adjust the filter to reject the interfering signal and pass the desired signal.
In a preferred embodiment, the filter includes multiple cascaded delay elements that are responsive to the symbols so the delay elements contain values representing the amplitudes of successive received symbols. An electronic device combines signal values representing the values in the elements with signal values representing the errors to derive an output signal for each of the delay elements. An electronic device combines values representing the output signals of each delay element to derive the filter output.
In one embodiment, the desired signal includes orthogonal I and Q channels, which include sequential symbols resulting from simultaneous I and Q components. The filter includes multiple delay intervals. In the filter, the I and Q components are delayed by multiple successive equal time intervals equal to an integral multiple of the interval between adjacent symbols. In the illustrated embodiment, the amplitude of each symbol is sampled once so the multiple is one. However, the invention is not so limited and there can be two or even more samples per symbol; if there are two samples per symbol the multiple is two, etc. for larger numbers of samples per symbol. Signals determined by the sampled amplitudes of the delayed I and Q components, as delayed for corresponding times, and signals representing the values of the I and Q errors are outputs of 78-78.6 combined to derive an I and a Q output signal for each of several delay intervals. The I output signals for each of the several delay intervals are combined as are the Q output signals for each of the several delay intervals. The I errors are derived by responding to the combined I output signals and the I estimates while the Q errors are derived by responding to the combined Q output signals and the Q estimates.
In accordance with another aspect of the invention, there is provided a method of terrestrially receiving a first electromagnetic wave having a predetermined bandwidth emitted from a first geosynchronous satellite that is spaced from a second geosynchronous satellite by a predetermined relatively small arc. The second geosynchronous satellite emits a second electromagnetic wave having (a) a narrower bandwidth than the first wave, (b) frequency components in the bandwidth of the first wave, and (c) a carrier frequency that is subject to change. The first and second waves are simultaneously emitted by the first and second satellites.
The method comprises simultaneously transducing the first and second waves into first and second simultaneously derived electrical signals that are replicas of variations of the waves. The waves are transduced by a terrestrial antenna pointed generally at the first satellite and having a beamwidth for receiving the first and second waves with about the same amplitude. In response to the first and second signals, the second signal is substantially attenuated relative to the first signal so the first signal is passed to an output device. The output device produces a perceptible output in response to the first signal and fails to produce a perceptible output in response to the second signal. The substantial attenuation of the second signal is preferably provided by discriminating its bandwidth from that of the first signal.
In accordance with another aspect of the invention, there is provided an apparatus for receiving wideband emissions from a first geosynchronous satellite in the presence of narrow band emissions from a second geosynchronous satellite having a relatively close arcuate spacing from the first satellite. The wide band and narrow band emissions are capable of having overlapping spectra and the narrow band emission has a carrier frequency subject to change. The apparatus comprises an antenna having a beamwidth such that emissions from the first and second satellites are respectively transduced into first and second simultaneously occurring electrical signals having approximately the same amplitude. An adaptive filter responsive to the first and second electrical signals substantially attenuates the second signal relative to the first signal so the first signal, as coupled to an output terminal of the filter, has an amplitude that can produce a perceptible response in an output device and the second signal, as coupled to the output terminal, has an insufficient amplitude to produce a perceptible response in the output device.
Preferably the filter includes means responsive to the first and second signals for passing most frequencies in the bandwidth of the first signal and for substantially attenuating substantially all frequencies in the bandwidth of the second signal. The means for passing and substantially attenuating preferably performs a statistial analysis, based on minimum least mean square error techniques, of the symbols included in the filter output signal and the first input signal.
The means for passing and substantially attenuating preferably includes a delay line having multiple taps on which are derived signals that are combined to provide the filter output. Signal processing means controls the amplitude of signals derived from the taps that are combined.
An embodiment of the present invention uses adaptive optimal linear filtering to reject unwanted interference within the bandwidth of the desired signal. The unwanted interference frequently has a carrier frequency subject to variation, such that the interference is subject to having a carrier frequency that moves back and forth across the spectrum of the desired signal. The bandwidth of the unwanted interference may be as great as 25 percent of the bandwidth of the desired signal. The interference can be from a coherent or incoherent source and is considered to be a quasi stationary process. It is necessary for the filter, which is based on the minimum least mean square error principle, to be adaptive because of the tendency of the carrier frequency of the interference to vary relative to the frequency of the desired signal.
The specific adaptive optimum linear filter is similar to a prior art filter disclosed by Proakis in the book Digital Communications, 2nd Edition, McGraw-Hill 1989. The filter disclosed by Proakis is known to reduce intersymbol interference in band limited channels, in particular to reduce the adverse effects of reflections in terrestrial television transmission. The present invention recognizes that signal processing structures similar to those disclosed by Proakis can be employed to discriminate against interference from interfering sources having spectral content as described above. In the application of the present invention, the adaptive linear filter is forced to converge to a filter that minimizes the mean squared error between the output of the filter and the desired signal.
Embodiments of the invention are most ideally suited for reception of phase shift key signals containing television programming information as emitted from a direct broadcast satellite in geosynchronous orbit; the phase shift key modulation can be of any form, particularly quadrature phase shift key and bi-phase shift key. The invention can also be employed in connection with cable networks, wherein a quadrature amplitude modulated (QAM) signal having a wide bandpass and containing digitally modulated television broadcast information and conventional television broadcast signals containing analog video share portions of the same bandwidth. The QAM modulation can be of any of the recognized forms, including the 64- and 256-ary forms. The invention can also be employed in situations wherein conventional terrestrial analog television broadcast information shares a portion of the bandwidth of a terrestrial multi-level vestigial sideband signal proposed for high definition television broadcast.
Because the spectrum of the desired signal differs from that of the interference, a linear filter designed to respond to an error between the output of the filter and a true value of the signal causes the error to be driven to the lowest value achievable to provide an acceptable signal to noise ratio gain. This is because the filter optimally discriminates against the interference on the basis of the frequency of the interference. In essence, the adaptive linear filter creates narrow band frequency notches to substantially reduce the energy of the interfering signal, as coupled to the filter output.
The filter must be provided with the true value of a transmitted signal or a surrogate of the desired signal that is close to the true value of the transmitted signal. Thus, either a known signal is transmitted while the filter is in a training mode or the filter must be able to estimate the desired signal with a reasonable level of confidence. The filter is in a training mode when parameters thereof are being modified to enhance interference rejection. If the filter is responsive to a television signal from a direct broadcast satellite, in which case a training signal cannot be employed, the desired signal ray be stronger than the interference or may differ from the interference in modulation format or carrier frequency or both. Therefore, a hard decision based on the output of the filter and on operator knowledge of the characteristics of the desired signal should be close enough to the desired signal to provide training for the adaptive filter.
Hence, the present invention is concerned with an adaptive filter to reject in-band interference due to other signal sources. Many alternatives for the configuration of the filter and for the process to set the filter parameters are possible. However, any adaptive process for controlling the filter must be supplied with a replica of the desired signal that is close to being correct during a period when the parameters of the filter are being modified.
In the preferred embodiment, the adaptive interference rejection filter is configured so it has a finite impulsive response having values corrected after a hard decision has been made for each transmitted channel symbol. The adaption method is known as the Widrow Hoff least mean square algorithm, disclosed in the book Adaptive Signal Processing by Widrows et al. A description of this technique in the context of adaptive equalization is provided in Proakis, ibid.